The present invention relates in general to a controlling device for use in a battery-powered vehicle, and more particularly to a speed control device for use in a battery-powered vehicle adapted to control a shunt field current in a compound electric motor thereby to obtain the speed control of a battery-powered vehicle.
In a general battery-powered vehicle application, unlike an ordinary electricity-powered vehicle which is continuously supplied with electricity from an external source of power, it is inevitable that there is practically a substantial restriction in a vehicle's capability such as a distance or a payload that the vehicle can cover with a single charge of its battery. In consideration of this fact, it is essential in the design of such a vehicle to obtain an as high as possible operating efficiency so as to have a maximum extent of its capability in terms of a running distance or a payload. In recent years, there was developed the direct current chopper system, and the operating efficiency of such a battery-powered vehicle has been improved substantially with the introduction of such direct current chopper system to the battery-powered vehicle, instead of the conventional speed control system incorporating the series-connected resistance and switches. However, with the application of the direct current chopper system, because of an increase in the iron (core) loss produced from the chopping of a voltage applied, the ohmic loss due to a great chopping current, etc. in comparison with a speed control system applying the shunt field control, a battery-powered vehicle having such direct current chopper system is still suffering from relatively low efficiencies in the starting and power running conditions. In this consideration, in the case of a battery-powered folk lift truck in which frequent starting and stopping operations are involved in the field duties or in the case of a battery-powered locomotive in which uphill and downhill operations are frequently encountered, it is actually an indispensable technique to put the vehicles in the regenerative braking condition so as to convert the kinetic energy of the train into the electric energy for improving the overall operating efficiency of the vehicles. However, in the practical design and construction of such the power regenerating installation of such direct current chopper systems, which have been practiced in the ordinary external-power-sourced electric trains, the circuitry and devices incorporated therein inevitably become complicated, thus resulting in a more initial cost and maintenance expenses, which has not been practiced particularly in the application to the fork lift trucks in which frequent starting and stopping operations are essential in their routines.
In addition, in the case of the chopper system, when a voltage in the armature of an electric motor becomes higher than that of a power source, a current in the armature is caused to overflow in the windings thereof, thus leading to an uncontrollable state, and consequently, there is such a defect that it is practically impossible to obtain a sufficient efficiency in the regenerative braking operation for the reason that in a high revolutionary range of the armature where a maximum energy can be regenerated, the current passing through the armature must necessarily be limited to a relatively low level.
In consideration of such drawbacks of the conventional art as mentioned above, it would be advantageous if an alternative may be realized to overcome such drawbacks. The present invention is essentially directed to meet such requirements, in which there is provided a unique circuitry having a reactor connected in series with a compound motor, which can be adapted to control the shunt field, thereby to obtain a speed control of the electric motor or a vehicle.
Incidentally, like the regenerating reactor included in the conventional system of the chopper type, in the practice of a speed control by way of the shunt field control according to this invention, a reactor for the purpose of preventing a sharp current rise in the starting or an irregular surge in the armature is needed as mentioned above. This reactor is an essential component of this invention for achieving a simple-construction yet high-efficiency regenerative brake system. In a series of experiments, when a 5 KWH electric motor is applied to the driving of a fork lift truck having a light weight of 3 to 4 tons, it is sufficient to use a reactor having a specific conductance of 20 milli henries or so. However, from the actual design and constructional viewpoint, the space, weight and cost of such a reactor would be a substantial obstacle in the production of a vehicle. In consideration of this fact, this invention is intended to overcome such obstacles as mentioned above.